1. Field of the Invention
The present invention relates to an impact energy absorbing member which is provided at the interior portion of an automobile door and absorbs impact energy at the time of a side collision. In particular, the present invention relates to an impact energy absorbing member in which, when the impact energy absorbing member receives an impact load, resin ribs plastically deform so as to absorb the impact energy.
2. Description of the Related Art
A structure has conventionally been known in which an energy absorbing member is provided in the side door of a vehicle such as an automobile for times when the side door receives an impact load from the vehicle transverse direction due to a side collision or the like. For example, Japanese Utility Model Application Publication (JP-Y) No. 5-410, Japanese Utility Model Application Laid-Open (JP-U) No. 3-13251, JP-U No. 3-49110 and JP-U No. 3-49111 disclose methods of absorbing the impact due to load at the time of a side collision by disposing an impact absorbing member formed from foamed urethane, foamed styrene or the like within a side door separately from the parts of the door.
JP-U No. 4-128912, Japanese Patent Application Laid-Open (JP-A) No. 6-72153, JP-U No. 6-78035, JP-A No. 6-247199, and JP-U No. 7-31432 disclose various impact absorbing members which absorb impacts by resin members which are formed by a plurality of ribs. Further, JP-U No. 7-13533 discloses an impact absorbing body having a hollow, box-shaped structural body which is mounted to the inner side of the door trim with an opening thereof oriented toward the door inner panel. The side wall portions of this structural body are inclined with respect to the direction in which the opening opens. JP-A No. 7-52735 discloses forming hollow bosses at the points of intersection of lattice-shaped ribs provided at the reverse surface of the door trim. In JP-A No. 7-228144, lattice-shaped resin ribs, in which slits for adjusting compression/buckling are formed at one side or both sides of the ribs, are provided at the reverse surface of a door trim base plate. JP-A No. 7-232556 discloses providing a gap between the inner panel and the distal ends of ribs which extend perpendicularly from the door trim, and form cut-out portions in the centers of the rib distal ends. In this way, impact absorption is carried out in two stages.
These impact absorbing members absorb impact energy due to the plastic deformation of ribs which receive a load due to the impact. By molding the impact absorbing members from resin, the ribs can be molded integrally with, for example, the door pocket provided at the side door, so that the number of processes for assembling the side door can be decreased.
FIG. 6 illustrates an ideal load-displacement amount curve (withstand load characteristic) of an impact absorbing member at the time of impact energy absorption. In an impact absorbing member having an ideal withstand load characteristic, immediately after the impact absorbing member receives the impact load and begins to deform plastically, the load increases in accordance with the amount of displacement. However, when the load reaches a predetermined value and thereafter, variations in the load are suppressed, and only the amount of displacement changes. The energy absorption amount of the impact absorbing member is expressed by the area between the load-displacement amount curve and the horizontal axis. The closer the configuration of the region between the load-displacement amount curve and the horizontal axis is to a rectangle, the greater the area of this region. The ideal withstand load characteristic is substantially constant, without a sudden increase in the load, and has a low load value.
The withstand load characteristic needed for the impact absorbing member differs in accordance with the conditions of usage, the structure of the side door, the parts which are used, and the like. It is known that, when ribs are used as impact absorbing members, the withstand load characteristic varies greatly in accordance with the thickness of the ribs, the size of the ribs, and the intervals (spacings) at which the ribs are disposed. In particular, when the ribs are formed by injection molding or the like, drafts must be formed at the ribs in order to remove the molded product from the mold. Accordingly, the ribs are formed in taper shapes with the distal ends thereof being thin, and the proximal base portions thereof being thick. Although it is easy for the distal ends of such ribs to deform plastically, it is difficult for the proximal base portions thereof to deform plastically.
Therefore, when the impact absorbing member is being manufactured, in order to obtain a desired withstand load characteristic, the thickness of the ribs, the size of the ribs, the intervals (spacings) at which the ribs are disposed and the like are determined. A mold is made in accordance with the determined dimensions, and resin molding is carried out.
However, in a case in which the desired withstand load characteristic is not obtained, it is necessary to change the thickness of the ribs, the size of the ribs, the pitches, or the like. Because such changes affect the configuration of the mold to be used for molding, it is necessary to manufacture a new mold. Through such a process, manufacturing an impact absorbing member which results in a desired withstand load characteristic is extremely difficult. The costs involved in manufacturing the impact absorbing member increase, and a long period of time for production is required from the trial manufacturing stage until the mass production stage.